1. Field of the Invention
The present invention relates to a drive force distribution control system for use with a four-wheel drive motor vehicle, and more particularly to a drive force distribution control system for use with a four-wheel drive motor vehicle, for determining a ratio between drive forces to be distributed to front and rear wheels based on both the drive force transmitted to one of the front and rear wheels and the slip ratio between said one wheel and the road surface on which the motor vehicle is running.
2. Description of the Relevant Art
Four-wheel drive motor vehicles in which both front and rear wheels are drivable generally have a transfer clutch for distributing and transmitting engine power to the front and rear wheels to enable them to exert drive forces for propelling the motor vehicle.
As disclosed in Japanese Laid-Open Patent Publication No. 62-6831 (published: Jan. 13, 1987), one recent four-wheel drive motor vehicle employs, as such a transfer clutch, a differential mechanism or a hydraulic multiple-plate clutch having a limited-slip differential function. The slip ratio between the front and rear wheels based on a reference rotational speed of one of the front and rear wheels is calculated according to a certain formula, and a ratio between drive forces to be distributed to the front and rear wheels is determined on the basis of the calculated slip ratio. The drive forces are then distributed to the front and rear wheels based on the determined distribution ratio. By thus controlling the drive force distribution, the steering characteristics of the motor vehicle can be stabilized to prevent the motor vehicle from excessively understeering during high-speed travel without lowering starting and accelerating capabilities of the motor vehicle.
It is known in general that a four-wheel drive motor vehicle tends to understeer by selecting a drive power distribution ratio such that the drive force distributed to rear wheels is smaller than the drive force distributed to front wheels. It is also known that the four-wheel drive motor vehicle has an oversteering tendency when running on a road having a low coefficient of friction with respect to vehicle road wheels because of low friction between the wheels (or their tires) and the road. Therefore, it is considered to be possible to prevent the four-wheel drive motor vehicle from oversteering and to give the motor vehicle good maneuvering stability by distributing a smaller drive power to the rear wheels when the motor vehicle is running on a road having a low coefficient of friction. However, it is difficult to directly and accurately measure the condition of the road on which an automobile is running in order to detect the coefficient of friction between the road wheels and the road. There has recently been proposed to employ an ultrasonic sensor or the like for indirectly measuring the road condition for the control of the drive force distribution. The ultrasonic sensor is however expensive and unreliable as it cannot accurately detect the coefficient of friction.